The invention relates in general to antenna constructions in radio apparatuses. In particular the invention relates to an antenna construction which has two resonating frequences different from each other. This patent application uses a mobile phone as an example of a radio apparatus.
In different parts of the world there are cellular telephone systems in use that differ from each other significantly in their operating frequency ranges. Of digital cellular telephone systems, the operating frequencies of the Global System for Mobile Tele-communications (GSM) are 890-960 MHz, those of the Japanese Digital Cellular (JDC) system in the 800 and 1500 MHz bands, of the Personal Communication Network (PCN), 1710-1880 MHz, and of the Personal Communication System (PCS), 1850-1990 MHz. The operating frequencies of the American AMPS mobile phone system are 824-894 MHz and those of the Digital European Cordless Telephone (DECT) system, 1880-1900 MHz.
Since the resonating frequency of a prior-art radio-frequency antenna is in a known manner connected with the length of the antenna, through the wavelength, a certain antenna can be used only in a mobile phone designed for a cellular system of a certain frequency area. In some cases, however, it is desirable that one and the same phone could be used in some other frequency range, too. In addition to other suitable RF parts, a working antenna arrangement is then needed.
U.S. Pat. No. 4,442,438 discloses an antenna construction resonating at two frequencies, substantially comprising, as shown in FIG. 1, two helixes HX1, HX2 and one whip element P1. The helixes HX1 and HX2 are positioned one after the other along the symmetry axis of the construction, and their adjacent ends A1 and A2 constitute the feed point of the combined construction. The whip element P1 is partly inside the upper helix HX1 extending to some extent outside it, and its feed point A3 is at its lower end. A RF signal is brought to the feed point A3 via a coaxial conductor KX coinciding with the symmetry axis of the construction and travelling through the lower helix HX2. The feed point A3 of the whip element is coupled to the lower end A1 of the upper helix, and the lower helix is coupled at its upper end A2 to the conductive and earthed shroud of the coaxial conductor KX. The construction's first resonating frequency is the resonating frequency of the combined structure of helixes HX1 and HX2; 827 MHz in the illustrative embodiment. The second resonating frequency of the construction is the common resonating frequency of the upper helix HX1 and the whip element P1; 850 MHz in the illustrative embodiment. Thus, helix HX1 and whip element P1 are dimensioned such that they have substantially the same resonating frequency.
The construction disclosed by the U.S. Patent is relatively complex. From the manufacturing standpoint, the most difficult part in the construction is the feed point arrangement at the middle of the antenna, where the lower end A3 of the whip element and the lower end A1 of the upper helix have to be galvanically coupled, and the lower helix has to be coupled at its upper end A2 to the shroud of the coaxial conductor feeding the whip element. According to the material presented in the patent the difference between the two resonating frequencies achieved by the construction is small because the upper helix HX1 and the whip element P1 have to be dimensioned such that they have substantially the same resonating frequency, so the construction cannot be applied to a phone operating at the GSM and PCN frequencies, for example. Indeed, in the description of the patent it is stated that an object of the invention is to broaden the resonating frequency area of the mobile phone antenna such that it would better cover the whole frequency range in one cellular system.
Finnish patent application FI 963275 (LK-Products) discloses a dual-frequency antenna construction according to FIG. 2 in which there is at a certain point between the ends of a helix antenna 2 wound into a cylindrical coil a coupling part 2c for the coupling to a second antenna element 3. The cylindrical coil conductor 2, which is the first antenna element in the antenna, comprises in the direction of its longitudinal axis a lower part 2a and an upper part 2b, and the second antenna element 3 is connected to the cylindrical coil conductor through a fixed coupling at the coupling point 2c between the lower and upper parts. The two radiating antenna elements of the construction have a common lower part up to the branching point consisting of the coupling part, from which point on the electrical lengths of the antenna elements are different. The first resonating frequency of the combined antenna construction is determined by the total electrical length of the common lower part of the antenna elements and the upper part of the first antenna element. The second resonating frequency is determined by the total electrical length of the common lower part of the antenna elements and the upper part of the second antenna element. In addition, the resonating frequencies are affected by the mutual coupling of the antenna elements and the fact that the antenna elements are electrically conductive bodies in the near fields of one another so that they put a load on each other. The antenna. construction according to FIG. 2 is relatively difficult to precisely dimension to the desired frequencies since the coupling point between the antenna elements requires quite accurate positioning. In addition, the electrical coupling in the coupling point easily becomes unreliable.